Fix #280

Since `parent` is not really used as a parent, I use it as the last
visited encrypted part while going down the parts-tree.

At the decryption of a part (`mu_msg_crypto_decrypt_part`) I check,
through the GMimeDecryptResult, for signatures (`check_decrypt_result`)
and add them to the part (`tag_with_sig_status`).  Any nested parts hold
that encrypted part as their parent.  Finally at `handle_part`, for each
part I check if it a descendent of an encrypted part.  If so, I proceed
checking for signatures and adding them to the `msgpart`.

lib/mu-msg-part.h

@@ -25,6 +25,8 @@
 #include <glib.h>
 #include <unistd.h> /* for ssize_t */
 
+#define SIG_STATUS_REPORT "sig-status-report"
+
 G_BEGIN_DECLS
 
 enum _MuMsgPartType {